The present invention relates to the art of image and video processing. It finds particular application in conjunction with the processing and display of medical diagnostic images and will be described with particular reference thereto. However, it is to be appreciated that the present invention may also be applicable to other environments in which image data is selectably mapped rather than being displayed directly.
CT scanners and other medical diagnostic imaging apparatus reconstruct images which are defined by a rectangular array of pixel values. Each pixel value is determined with a relatively high resolution, e.g. 14 bits or more. However, conventional video monitors have a much smaller displayable gray scale resolution, commonly eight bits or 256 gray scale levels.
Map values for mapping the higher resolution pixel values into the lower resolution video values are commonly stored in a look-up table. The look-up table is a random access memory device having an address input with the same number of bits as the pixel value resolution, e.g. 14, and a data output with the same number of bits as the video signal resolution, e.g. 8.
Commonly, the medical diagnostic image processors have an operator adjustable digital filter or mapping function. More specifically, there is a continuously adjustable level function which determines which value of the 14 bit input address provides the center gray scale value. A window adjustment selects the number of pixel value bits which are expanded, contracted, or otherwise mapped into the gray scale. That is, rather than mapping a selected 256 of the 16k pixel levels into the 256 video monitor gray scale levels, a lesser number, e.g. 128 levels, may be expanded into the 256 gray scale levels or a larger number, e.g. 1024 pixel levels, may be compressed into the 256 video monitor gray scale levels. Moreover, the relationship between the pixel and video values may be linear or non-linear, as may be selected by the operator. For example, in a monochromatic display system, those bits above the selected window are designated white, and those below the selected window are designated black. By adjusting the level and window functions, the gray scale can be defined by the most significant bits of the pixel values, the least significant bits, or any bits in between.
These window and level functions are commonly continuously adjustable in real time by the operator in order to define a gray scale which best illustrates the anatomical portion of the displayed image that is of primary diagnostic value. Each time the operator changes the window or level function, the mapping values in the 8 bit wide look-up table are changed. In the prior art equipment, a central processor would read a 32 bit longword (4 bytes) of new map values from a memory means in a first memory cycle. In a second memory cycle, the central processing unit would address the 8 bit wide look-up table and provide the first byte of new mapping values. In a third time period, the central processing unit would provide the next address to the look-up table along with the second byte of map values. In a fourth memory cycle, the CPU would address the look-up table with a third address as it transfers a third byte of the mapping data. Finally, in a fifth memory cycle, the central processor would address the look-up table and transfer the fourth byte of mapping data. In this manner, five memory cycles are required to reload four of the addresses of an 8 bit wide look-up table.
One of the problems in the medical diagnostic image field is that the operator conventionally views the image as the mapping or filter function is adjusted. The operator expects to obtain immediate visual feedback of the adjustment. Because image data is being moved through the same look-up table to create the image, the mapping function value can only be reloaded when no image data is passing through the look-up table, e.g. during the vertical blanking period. Because the vertical blanking period is much shorter than the time necessary to reload the entire new mapping function into practical size look-up tables, the function is reloaded piecemeal over several video frames. This causes a perceptible delay in the change of the mapping function and a readily perceivable distortion during the displayed image frames in which the mapping function is partially replaced.
In accordance with the present invention, a new and improved method and apparatus are provided for speeding up the process of look-up table loading such that completely reloading the filter function in a practical size look-up table within a single vertical blanking period is possible. Of course, larger, more complex look-up tables with greater numbers of address bits and more filter values can also be loaded over the course of a lesser number of video frames than previously possible.